Various kinds of crafts and buildings (for example, aircrafts, windmills, automobiles, and the like) may be struck by lightning. It is very important to measure a current (hereinafter, referred to as a “lightning current”) generated by a lightning strike or a current distribution in anti-lightning design.
For example, as specified in SAE standard ARP5412.revA (see FIG. 7), the lightning current includes a large current (a region A of FIG. 7) which has a maximum peak equal to or greater than 100 kA (about 200 kA at the tip of the main wing of an aircraft and about 100 kA at the center of the main wing) and a pulse waveform from the rise to 500 μsec, a current (a region B of FIG. 7) which is 2 kA on average and gradually decreases to about 5 msec, and a small current (a region C of FIG. 7) which is 400 A on average and is maintained at about 1 sec. In this way, the lightning current is a current waveform which has a short duration but has a high dynamic range and a wide time band, and there is demand for accurately measuring the current value for anti-lightning design.
As general means for measuring the lightning current, means for measuring an electromotive force at a loop end caused by a temporal change of a magnetic flux, such as a current transformer or a Rogowski coil, is known.
However, in the above-described means, since the electromotive force at the loop end caused by the temporal change of the magnetic flux is measured, a low-frequency component and a DC component cannot be measured. In a measurement using a Rogowski coil, since capacitive coupling occurs according to construction circumstances to an object struck by lightning, an error is superimposed on an original signal by lightning. In a measurement in a high voltage and high current field, such as a lightning current, insulation treatment or a noise countermeasure for measurement means is important but is not easily implemented. In the above-described measurement means, since the dimension of a sensor for measuring a current is large to be equal to greater than ten-odd mm to tens of mm, the shape of the sensor cannot be significantly changed, and it is difficult to measure a narrow strain portion of about several mm, it is difficult to evaluate the path of the lightning current in detail.
Accordingly, means for measuring a current using an optical fiber current sensor capable of measuring a narrow strain portion is known. The optical fiber current sensor measures a magnetic field caused by a current using a photomagnetic effect and obtains a current value. The optical fiber current sensor evaluates change in Faraday rotation angle due to a photomagnetic effect of fiber light to obtain a current value.
As an example of the optical fiber current sensor, a system which obtains change in optical phase difference in a circular optical path, called a Sagnac effect, to measure a DC component of a current has been developed.
PTL 1 discloses a lightning current measuring method which supplies measurement light to a photomagnetic field sensor provided in an object struck by lightning, changes the rotation angle of the plane of polarization of measurement light with change in magnetic field generated around a lightning current flowing through the object struck by lightning in the photomagnetic field sensor, substitutes the rotation angle of measurement light output from the photomagnetic field sensor to light intensity, and measures the light intensity to measure a lightning current.
PTL 2 discloses a fiber current sensor in which a polarized light separation element separates output light from a sensor fiber into two polarization components having orthogonal planes of polarization, the two separated polarization components are respectively converted to a first signal Px and a second signal Py through photoelectric conversion, and a signal processing unit multiplies the ratio Sx of a DC component and an AC component of the first signal Px and the ratio Sy of a DC component and an AC component of the second signal Py by different coefficients to calculate the difference value thereof.